1. Field of the Invention
The present invention relates to a device for manufacturing a semiconductor device having a circuit constituted by a thin film. For example, the present invention relates to a device for manufacturing an electrooptical device typified by a liquid display device and the constitution of an electric device having the electrooptical device as a part. In this connection, in the present specification, a semiconductor device designates in general a device capable of functioning by the use of semiconductor characteristics and includes the above electrooptical device and electric device.
2. Description of the Related Art
In recent years, research and development have been widely conducted on the technologies for performing a laser annealing processing to an amorphous semiconductor film or a crystalline semiconductor film (semiconductor film which is not a single crystal but a polycrystal or a micro-crystal), that is, non-single crystal semiconductor film formed on an insulating substrate such as a glass substrate or the like to crystallize the non-single crystal semiconductor film or to improve its crystallinity. A silicon film is often used as the above semiconductor film.
A glass substrate has advantages that it is cheap and has good workability and is easy to make a large area substrate in comparison with a quartz substrate which has been conventionally used. This is because the above research and development have been carried out. Also, it is because the melting point of the glass substrate is low that a laser is widely used for crystallizing the semiconductor film. The laser can apply high energy only to a non-single crystal film without increasing the temperature of the substrate too much.
The crystalline silicon film is called a polycrystalline silicon film or a polycrystalline semiconductor film because it is made of many crystal grains. Since the crystalline silicon film subjected to a laser annealing processing has high mobility, a thin film transistor (hereinafter referred to as TFT) is formed by the use of the crystalline silicon film and, for example, is widely used for a monolithic liquid crystal electrooptical device having a glass substrate on which TFTs for driving a pixel and for a driving circuit are formed.
Also, a laser annealing method of transforming the high-power laser beam of a pulse oscillation such as an excimer laser into a square spot several cm square or a linear beam 10 cm or more in length at an irradiate surface by the use of an optical system and of scanning a semiconductor film with the laser beam (or moving a spot irradiated with the laser beam relatively to an irradiate surface) has been widely used because it increases mass productivity and is excellent in an industrial view point.
In particular, when a linear laser beam is used, the whole irradiate surface is irradiated with the linear laser beam only by scanning the irradiate surface in the direction perpendicular to the direction of the line of the linear laser beam, which therefore produces high mass productivity. In contrast to this, when a spot-like laser beam is used, the irradiate surface needs to be scanned with the laser beam in the back-and-forth direction and in the right-and-left direction. The irradiate surface is scanned with the linear laser beam in the direction perpendicular to the direction of the line of the linear laser beam because the direction is the most efficient scanning direction. The method of using the linear laser beam into which the laser beam emitted from the excimer laser of pulse oscillation is transformed by the use of a suitable optical system for the laser annealing processing has become a mainstream technology.
In FIG. 1 is shown an example of the constitution of an optical system for transforming the cross section of the laser beam into a linear shape at an irradiate surface. This constitution not only transforms the cross section of the laser beam into the linear shape but also homogenizes the energy of the laser beam at the irradiate surface. In general, an optical system homogenizing the energy of the beam is called a beam homogenizer.
The side view is explained first. A laser beam leaving from a laser oscillator 101 is partitioned in a direction perpendicular to the movement direction of the laser beam by cylindrical array lenses 102a and 102b. This direction is referred to as a vertical direction throughout this specification. There are four partitions with this structure. The partitioned laser beams are once collected into a single laser beam by a cylindrical lens 104. This is then reflected by a mirror 107, and once again condensed into one laser beam on an irradiation surface 109 by a doublet cylindrical lens 108. The doublet cylindrical lens refers to a lens composed of two cylindrical lenses. The linear laser beam is thus given energy uniformity in the width direction, and the length of the width direction is thus determined.
The top view is explained next. The laser beam leaving from the laser oscillator 101 is partitioned in a direction perpendicular to the movement direction of the laser beam, and perpendicular to the vertical direction, by a cylindrical array lens 103. This direction is referred to as a horizontal direction throughout this specification. There are seven partitions with this structure. The laser beams are next made into a single beam on the irradiation surface 109 by the cylindrical lens 104. The linear laser beam is thus given energy uniformity in the longitudinal direction, and the length is thus determined.
The above lenses are manufactured by synthetic quartz in order to respond to the excimer laser. Further, coating of the lens surface is performed so as to make it very transmissive to the excimer laser. The transmissivity of the excimer laser by one lens thus becomes equal to or greater than 99%.
By performing laser annealing on the entire surface of a non-single crystal silicon film by irradiating the linear laser beam, processed by the above constitution, while gradually shifting it in the width direction, crystallization can be performed and crystallinity can be increased.
A model method of manufacturing a semiconductor film which becomes an irradiation object is shown next. First, a 5 inch diagonal Corning 1737 substrate having a thickness of 0.7 mm is prepared. A SiO2 film (silicon oxide film) of 200 nm thickness is deposited on the substrate by using a plasma CVD apparatus, and an amorphous silicon film (hereafter referred to as an a-Si film) of 50 nm thickness is formed on the surface of the SiO2 film.
The substrate is heated for 1 hour at a temperature of 500° C. in a nitrogen atmosphere, decreasing the hydrogen concentration within the films. The laser resistance of the film is thus significantly increased.
A Lambda Corp. XeCl excimer laser (wavelength 308 nm, pulse width 30 nm) L3308 is used as a laser apparatus. The laser apparatus is a pulse emission type, and possesses the capability of delivering energy of 500 mJ/pulse. The size of the laser beam is 10×30 mm (both values are half-widths) at the exit of the beam. The shape of a laser beam generated by an excimer laser is generally a rectangular shape, and expressed as an aspect ratio, is in the range of approximately 3 to 5. The strength of the laser beam shows a Gaussian distribution, in which its strength increases as it approaches the center. The size of the laser beam is transformed into a 125 mm×0.4 mm linear laser beam having a uniform energy distribution by an optical system possessing the structure shown in FIG. 1.
According to experiments performed by the applicant of the present invention, when the laser is irradiated on the above semiconductor film, the overlap pitch is most suitable at approximately 1/10 of the width (half width) of the linear laser beam. The uniformity of crystallinity within the film is thus increased. In the above example, the half width is 0.4 mm, and therefore the pulse frequency of the excimer laser is set to 30 hertz, the scanning speed is set to 1.0 mm/s, and the laser beam is irradiated. The energy density of the laser beam in the irradiation surface is 420 mJ/cm2 at this time. The method used here to crystallize the semiconductor film using the linear laser beam is an extremely general method.
If the pulse emission excimer laser beam is processed into a linear shape by an optical system such as the one stated above, and then if the linear laser beam is irradiated while scanning, on a non-single crystal silicon film, for example, then a polycrystalline silicon film is obtained.
A phenomenon of film striping running in the vertical and horizontal directions is conspicuous when observing the polycrystalline silicon film obtained. (See FIG. 2.)
The semiconductor characteristics differ with each of the stripes, and therefore if the striped state film is used when manufacturing an integrated driver and pixel (system on panel) liquid crystal display, a problem develops in which the strips are output to the screen as are. The stripes output on the screen are caused by non-uniform crystallinity in both a driver portion and a pixel portion. This problem is being improved by improving the film quality of the laser beam and the quality of the non-single crystal silicon film which becomes the irradiation object of the laser beam, and has been improved to such an extent that, depending upon the liquid crystal display manufactured, it does not become a problem. However, when manufacturing a liquid crystal display with higher definition and good characteristics, the above striping nonetheless becomes a problem. The present invention is for solving this problem.
The main reasons that the above striped pattern is generated are: energy diffusion near the edges in the width direction of the linear laser beam (expressing a state in which the energy is attenuated as the edge of the laser beam is approached); and non-uniformity of energy in the longitudinal direction of the linear laser beam. An energy diffusion region is defined throughout this specification as a region having an energy density equal to or less than 90% of the maximum energy density within the linear laser beam.
The energy diffusion near the edge in the width direction of the linear laser beam becomes a cause of the formation of the stripe pattern in a direction parallel to the longitudinal direction of the linear laser beam. Furthermore, the energy non-uniformity in the longitudinal direction of the linear laser beam becomes a cause of the formation of the stripe pattern in a direction orthogonal to the longitudinal direction of the linear laser beam.
In order to resolve the problem of non-uniform energy in the longitudinal direction of the linear laser beam, an increase in the number of cylindrical array lenses 102 and 103 is considered.
The number of partitions of the cylindrical array lenses in the above example is four vertical partitions and seven horizontal partitions, for a total of 28 partitions. Tests for increasing the uniformity of the laser anneal by increasing the number of partitions have been performed over many years. Some examples of such tests are given below.
The size of one cylindrical lens structuring the four partition cylindrical array lens is a width of 3 mm and a length of 50 mm, long and narrow, in the above example. These values, if expressed as an aspect ratio relating to the width and the length of one cylindrical lens, are 50/3, or approximately 16.7. On the other hand, the size of one cylindrical lens structuring the seven partition cylindrical array lens is a width of 7 mm and a length of 50 mm, relatively fat, in the above example. When expressed as an aspect ratio, this is 50/7, or approximately 7.1.
It is therefore easy to make the seven partition cylindrical array lens thinner from the viewpoint of a manufacturing technique. However, if the energy distribution of the linear laser beam obtained by the above 28 partitions is investigated in detail, it is understood that the energy near the centerline in the width direction of the linear laser beam clearly differs from the energy near the edges of the linear laser beam in the same direction. The energy, which possesses a higher energy, differs from every time optical adjustment is performed. Therefore, no matter how much the number of partitions of the seven partition cylindrical array lenses is increased, the energy distribution within the linear laser beam will not tend toward a uniform direction, and it will only have a non-uniform distribution in the width direction of the linear laser beam.
Based on the above considerations, the only way to manufacture an optical system through which a very uniform laser anneal effect can be expected is to increase the number of partitions of the four partition cylindrical array lens.
However, the lenses structuring the optical system are of synthetic quartz, which is difficult to process. Further, the cylindrical lens of 3 mm width and 50 mm length structuring the cylindrical array lens given in the above example has a shape which is extremely slender as a lens, and therefore a high degree of technical skill is required for independent manufacturing of the lenses.
The above cylindrical array lens is made into a cylindrical array lens by mutually joining the cylindrical lenses manufactured one by one. Or, the cylindrical array is made by exposure to high temperature after forming the array, causing unification. Therefore, each cylindrical lens is initially separate.
In order to give each of the cylindrical lenses sufficient strength and precision, it is necessary for the aspect ratio between the lens width and the lens length to be at least equal to or less than 20. This value is based on experience of the applicant of the present invention. For example, in addition to the above cylindrical array lens, by forming 8 cylindrical lenses having a length of 60 mm and a thickness of 2 mm, lining up the cylindrical lenses in the width direction and putting them into a frame, the formation of the cylindrical array lens can be performed, but the precision is completely insufficient. In addition, the directionality of the laser beams passing through each cylindrical lens becomes so scattered that it can be understood by the naked eye, and the energy uniformity of the linear laser beam obtained becomes worse than that of the example shown previously. This example has an aspect ratio of 30.
For example, the width of all of the cylindrical lenses contained in the cylindrical array lens 102 in the example of the optical system shown in FIG. 1 is halved, and the number of partitions is doubled to 8 partitions, then the aspect ratio of one cylindrical lens contained in the cylindrical array lens 102 becomes 33, becoming larger than that of the cylindrical array lens of 2 mm width formed earlier.
In order to increase the number of partitions without increasing the aspect ratio of the cylindrical lenses structuring the cylindrical array lens, a method of expanding the laser beam output from the laser oscillator by using a beam expander can be used, but if the aberration of the doublet cylindrical lens 108 is not reduced by the amount that the laser beam is expanded, then a new problem develops in which the laser beam does not sufficiently unify on the irradiation surface.
An example of specification of the doublet cylindrical lens 108 is shown below, in accordance with FIG. 7. The doublet cylindrical lens 108 has a focal distance of 175 mm, a width of 70 mm, and a length of 160 mm, with a center thickness of 31 mm. The above lens has curvature in the width direction. The radius of curvature of a laser beam incidence surface 701 is 125 mm, the radius of curvature of a next surface 702 is 69 mm, and the center distance between the surfaces 701 and 702 is set to 10 mm. One cylindrical lens can be made with this structure. A second cylindrical lens has a laser beam incidence surface 703 placed at a center distance of 1 mm away from the surface 702. The radius of curvature of the laser beam incidence surface 703 is 75 mm, and the radius of curvature of a next surface 704 is set to −226 mm. The center distance between the surfaces 703 and 704 is 20 mm. Symbols attached to the radius of curvature show the curvature direction.
An example of computing the spot size in the focal point of a doublet lens possessing a curvature similar to that of the doublet cylindrical lens 108 by using the optical design software Zemax is shown in FIGS. 12A and 12B. FIG. 12A shows the beam spot when parallel light with a wavelength of 308 nm and a diameter of 24 mm is incident on the above doublet lens. The spot size becomes approximately 50 μm. Therefore, when the laser beam is incident on the doublet cylindrical lens 108, if the width of the laser beam is assumed to be 24 mm, then it can be found that the diffusion in the width direction of the linear laser beam becomes on the order of 50 μm. The width of the linear laser beam made by the optical system in the above example is 400 μm, and therefore, the ratio of the above width to the width of the linear laser beam in the diffusion region exceeds a ratio of 10%. This diffusion becomes a cause of the formation of horizontal stripes in a silicon film.
On the other hand FIG. 12B shows the beam spot when parallel light with a wavelength of 308 nm and a diameter of 12 mm is incident on the above doublet lens. The spot size becomes equal to or less than 4 μm. This corresponds to diffusion on the order of 1% of the linear laser beam width. Demanding a higher precision than this is difficult from the standpoint of precision of lens manufacture.
From the above simulation results, it can be understood that in order to suppress the influence of the aberration of the doublet cylindrical lens 108, the size of the incident laser beam should be made as small as possible. Or, generally, the doublet cylindrical lens 108 must be replaced by an aspherical lens, or it must be replaced by a high precision lens equal to or better than a triplet lens structured by three lenses.
With present techniques, it is extremely difficult to ashperically process synthetic quartz. Further, forming a triplet lens is also not advisable from the viewpoint of cost or adjustment. The thinner the width of the laser beam generated by the oscillator is made, the more the aberration of the doublet cylindrical lens 108 can be suppressed, and therefore it is not preferable to make the size of the laser beam generated by the oscillator much greater than on the order of 10×30 mm (with 10 mm corresponding to the width of the laser beam).
Considering the above, it can be expected that, with the present structure, the number of partitions of the four partition cylindrical array lens can only be increased to around five partition. With this number of partitions, the energy uniformity of the linear laser beam will not change from the present state. Although it is understood that the surface area of a portion for the edge energy diffusion in the width direction of the linear laser beam increases, the current situation is that the number of laser beam partitions is increased by a method of expanding the width of the laser beam using a beam expander to secure the above uniformity.